Determining the Water Quality Index

In an attempt to devise a system to compare rivers and lakes in various parts of the country, the NationalSanitation Foundation (NSF) created and designed a standard index called the Water Quality Index(WQI). The WQI is one of the most widely used of all existing water quality procedures. The overallresults of nine separate tests can be used to determine if a particular stretch of river is healthy.The WQI consists of nine tests:Dissolved OxygenFecal ColiformpHBOD (Biochemical Oxygen Demand)TemperatureTotal PhosphateNitratesTurbidityTotal SolidsAfter completing the nine tests, the results are recorded and transferred to a weighting curve chart where anumerical value is obtained. For each test, the numerical value or Q-value is multiplied by a weightingfactor. For example, dissolved oxygen has a relatively high weighting factor (.17); because it is moresignificant in determining water quality than the other tests. The nine resulting values are then added toarrive at an overall water quality index (WQI). The highest score a body of water can receive is 100.Water Quality Index Ranges90-100 Excellent70-90 Good50-70 Medium25-50 Bad0-25 Very Bad

If you are unable to run all nine tests and you want to estimate the Overall WaterQuality Index, students could determine the Q value of missing data by examiningknown data. For example , if the fecal coliform test were not run, the results or Qvalue of three related tests (dissolved oxygen, nitrates, total phosphate) could beaveraged to get a score for that test. Using the table below as an example, the averageQ-value of the three tests is 62 - or a fecal coliform count of 20 colonies per 100 ml ofwater.

Calculating NSF Water Quality Index

Water quality index is a 100 point scale that summarizes results from a total of nine differentmeasurements when complete:TemperaturepHDissolved OxygenTurbidityFecal ColiformBiochemical OxygenTotal PhosphatesNitratesTotal Suspended SolidsUsing the book Field Manual for Water Quality Monitoring, the National Sanitation Foundationsurveyed 142 people representing a wide range of positions at the local, state, and national levelabout 35 water quality tests for possible inclusion in an index. Nine factors were chosen and somewere judged more important than others, so a weighted mean is used to combine the values.So that field measurements could be converted to index values, respondents were asked byquestionnaire to graph the level of water quality (0 through 100) corresponding to the fieldmeasurements (e.g., pH 2-12). The curves were then averaged and are thought to represent thebest professional judgment.Water Quality Factors and WeightsFactorWeightDissolved oxygen0.17Water quality index is a 100 point scale that summarizesresults from a total of nine different measurements Graphs are used to convert field data to a Q or QualityValue.The Q value is then multiplied by Weighing Factor to getthe Water Quality Index for that Chemical. The resultsare then totaled to get the Overall Water Quality Index

Fecal coliform

0.16

pH

0.11

Biochemical oxygen demand

0.11

Temperature change

0.10

Total phosphate

0.10

Nitrates

0.10

Turbidity

0.08

Total solids

0.07

Water Quality Index: Fecal Coli

Note: If the number of fecal coliform colonies is greater than 100,000,